Filtration is widely used throughout industry, commerce and domestic life. Filtration involves the physical separation of one or more components from a carrying fluid by passage of that fluid through or across a barrier that is permeable only to some of these components. Filtration can be distinguished in depth filtration and cake filtration. Using the depth filtration a particle can be trapped in the depth of the filter medium. Using the cake filtration a thick layer or cake of particles accumulates on the surface of the filter medium.
Depth filters separate coarse and fine haze substances and offer colloid retention rates while simultaneously preserving beneficial ingredients with low color retention. Gentle treatment of flavor and color plays a particularly important role in the filtration of spirits. In addition to reliable separation of fine colloidal and coarsely dispersed haze substances, the aim of filtration includes selective separation of haze-producing long-chain fatty acid esters. Depth filters with a low calcium and magnesium ion content as well as high fusel and essential oil absorbing capacity are used in particular for this purpose.
The production of mineral water, table water, and product water as well as industrial water places particularly high demands on filtration technology. Particular emphasis is placed on reliable particle separation by depth filtering. Depth filters assure safe and reliable filtration. There are multitudes of possible filtration applications in the food industry in which depth filters play a major role. Filtration processes are typically used in the production of gelatin, liquid sugar and invert sugar syrup, glucose, dextrose, extracts, essences, enzyme solutions, and rennin. Filtration is especially important, for example, in the production of edible oils where top priority is placed on retaining taste, vitamins, and the fatty acid composition while removing haze and mucilaginous substances.
U.S. Pat. No. 4,032,457 A discloses a filter cartridge wherein the filter has two or more distinctly different fibers layers with at least one of the fibers layers containing active particles. The different layers are formed in place on the filter by a wet accretion process. The filter has an inner annular core in the form of a perforated tube. The tube might be made of a plastic material such as polypropylene and is perforated with holes. The size of the holes may vary depending on the physical characteristics of the fibers to be accreted onto the tube. The tube functions to provide compressive strength to the filter element and defines a central flow passage through which the filter material is discharged after having passed through the filtering faces which have been accreted onto the tube.
DE 90 16 243 U1 discloses a filter module for filtering fluid media comprising a plurality of annular filter cells which are stacked. The filter cells have a circular ring-shaped form having an outer filter sheet and an intermediate mounted spacing element for keeping open an inner flow passage. The filter cells are stacked along a central filtrate passage element. By this arrangement with stacked filter cells the surface area of the filter medium is increased compared to a filter tube according to U.S. Pat. No. 4,032,457 A.
The filter sheets according to DE 90 16 243 U1 are manufactured in a papermaking process. The manufacturing of the filter sheets is a wet-laid papermaking process with all ingredients of the filter medium being suspended in water with a special mixer. The consistency of the suspension may range from 0.5 to 5.0%. The suspension (pulp) is dewatered on a Fourdrinier wire mesh. After dewatering, the flat paper is dried. The filter sheets are cut out from the dried paper and arranged onto the spacing elements in order to obtain filter elements which can be stacked. The dried paper can also be used for rolled filter modules and single use filter units.